System and method for geolocating plural remote transmitters

ABSTRACT

A system and method for locating plural remote transmitters in which many, relatively inexpensive transmitters may simultaneously send identifying signals to one or more receiving stations. The receiving stations detect the signals from the remote transmitters, determine the time of arrival of the signals and decode the information (if any) contained in the signals, all without synchronization between the receiving stations and the transmitters or among the transmitters and all without a central system for polling the various remote transmitters. The system combines the signal information received from two or more receiving stations to determine the geolocation of the transmitting units.

RELATED APPLICATIONS

This application is one of three related applications filed by thepresent inventor on even day herewith, all assigned to the sameassignee. The other two applications are: "A System and Method ForCommunicating With Plural Remote Transmitters," Ser. No. 08/708,031;and, "A System and Method For Communicating and/or Geolocating PluralRemote Transmitters Using A Time Invariant Matched Filter," Ser. No.08/708,030. Each of the other applications is herein incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention is related, generally, to systems and methods forcommunicating information and for locating the geolocation of a remotetransmitter. In particular, the present invention is related to a systemfor communicating from plural remote transmitters simultaneously withoutsynchronization or polling and for determining the geolocation ofspecific ones of the plural transmitters.

It is well known to deploy communications systems in which a single,relatively central base unit communicates with plural, often mobile,units within its transmission range. Central dispatch systems, such asthose employed by taxicabs, public services emergency personnel, and thelike communicate voice and, more recently, digital data between theremote units and the central base unit. Still other present systemscommunicate digital data only from the remote units to the central baseunit, such as "data radios" which often transmit remote sensor data froma wide geographic area to a central location. One known problem withsuch present systems is a need to coordinate the transmissions from thevarious remote units so as to avoid collisions with one another. Amyriad of techniques have been used to provide such coordination or toresolve collisions. For example, in some systems, the various remotetransmitters are polled by the central station in a logical or randomsequence. Only the remote unit receiving the poll is then permitted totransmit during a succeeding predefined time period. In other systems,each of the remote units receives a common clocking signal and transmitsto the central station at a time derived from the common clockingsignal. Both these prior art schemes avoid collisions to some extent butat a cost of requiring each remote unit to be capable of both receivingand transmitting signals. Additionally, the time taken to send outpolling and/or clocking signals reduces the amount of time available forthe remote units to transmit.

Other prior art systems avoid collisions by assigning separate transmitfrequencies for each remote unit, at an obvious cost of considerableamounts of bandwidth being occupied for systems having many remotetransmitters. Still other systems permit collisions to occur and relyupon the remote transmitters to monitor the communicating frequencies todetermine whether a collision has occurred and to reinitiatetransmission of the message upon the detection of a collision. Again,such systems require the remote units to have the capability of bothreceiving and transmitting signals. In addition, such systems aresometimes known to have repeated collisions, delaying the receipt of themessage contained therein, sometimes for relatively long periods oftime. Finally, the number of remote transmitters is limited in suchsystems to a number which is dependent upon how often each of the remoteunits transmits and the length of each transmission.

It is also known in the prior art to geolocate a remote transmittingunit based on multiple receipts of a signal emanating from a remotetransmitter. For example, plural receiving stations can receive a signaltransmitted from a remote mobile unit and compare the times-of-arrivalat the various receivers to determine a geolocation from which thesignal was transmitted. Similarly, plural receiving stations candetermine the angle-of-arrival of the signal from the remote mobile unitand by combining the different angle-of-arrival determinations at theplural stations can calculate the likely geolocation of the transmittingmobile unit. Generally, the complexity of such locating systemsincreases substantially with the number of remote units which must betracked.

As in the situation for voice or data communications, if plural remoteunits must be tracked simultaneously, the possibility of collisions oftheir signals increases. As the signals, collide, it is often difficult(if not impossible) for the receiver to distinguish the location of theunits sending the locating signals which have collided. To avoidcollisions, some prior art locating systems have used polling, commonclocking signals, individual frequencies, etc. so that the detectingreceiver can unambiguously detect a locating signal. Still other systemshave resorted to sending the locating signals multiple times to enhancethe possibility that the central receiver has received at least one ofthe locating signals. This latter technique obviously relativelyexpensive in terms of bandwidth.

It is also been proposed for locating systems to use direct spreadspectrum modulation techniques at the remote, mobile units to avoid theproblems of collisions and limited bandwidth availability. In suchsystems, each of the remote transmitters may use a Pseudorandom Number("PN") modulation to spread the locating signal across a wide bandwidth.Such systems are expected to meet with some success in the transmissionof the locating signals by plural remote units; however, the use of suchsystems usually involves the design and operation of a highly complexbank of correlators which can test incoming signals for the presence ofall the permitted PN codes in a very short period of time. A largenumber of possible PN codes usually results in a very complex andexpensive receiver to detect the various possible codes. If a systemuses plural receivers, as is often done to provide a relatively widegeographic range of coverage for a system, the costs of such relativelyexpensive receivers is multiplied by the need for plural such receivers.

Accordingly, it is an object of the present invention to provide a novelsystem and method for communicating simultaneously radio frequencysignals from plural, geographically diverse transmitters without acommon clocking signal or a polling signal.

It is another object of the present invention to provide a novel systemand method for communicating signals from plural transmitters without asubstantial number of collisions.

It is yet another object of the present invention to provide a novelsystem and method for communicating simultaneously from pluraltransmitters into a receiver without a bank of correlators.

It is still another object of the present invention to provide a novelsystem and method for determining the location of one or more of pluralsimultaneously transmitting remote units without a common clockingsignal or polling signal.

It is a further object of the present invention to provide a novelsystem and method for tracking the movement of plural remote units at arelatively low cost per unit.

It is yet a further object of the present invention to provide a novelsystem and method for communicating simultaneously from plural remotetransmitters without synchronization of the units to each other of tothe receiver(s).

It is still a further object of the present invention to provide a novelsignal processing architecture which simultaneously tracks the locationof plural transmitting units using a time invariant matched filter.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the art to which theinvention pertains from a perusal of the claims, the appended drawings,and the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified pictorial representation of a communicationsystem in which the present invention may be used.

FIG. 2 is a timing diagram illustrating an encoding technique which maybe used in a system of the present invention.

FIG. 3 is a graph illustrating a chirp signal which may be used in asystem of the present invention.

FIG. 4 is a simplified functional block diagram of a transmitter whichmay be used in a system of the present invention.

FIG. 5 is a simplified block diagram of a receiving station which may beused in a system of the present invention.

FIG. 6 is a simplified block diagram illustrating the operation ofdecoding scheme which may be used in the present invention.

FIG. 7 is a simplified block diagram of the channel processor portion ofthe receiving station of FIG. 5.

FIG. 8 is a timing diagram of the relationship among the subchannels ofthe channel processor of FIG. 7.

FIG. 9 is a pictorial diagram of the output of an FFT which may be usedin the channel processor of FIG. 7.

FIG. 10 is a pictorial diagram of the alignment of the output of the FFTof FIG. 9 for consecutive chirp signals.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1, a system in accordance with the presentinvention may include plural transmitters 10, which may be stationary ormobile, which are in communication with one or more receiving stationsor base stations 12. The receiving stations 12 may communicate with acentral station 14. This communication may be by way of conventionaltelephone circuits 13. The central station 14 may include a controlconsole 16, a storage unit 18, and means for communicating 20 with othercentral stations 14 or external systems. A common clock signal 22 may beprovided by a geostationary satellite system to each of the basestations 12.

In operation, the transmitters 10 may periodically or aperiodicallytransmit a beacon or signal to the base station(s) 12 within the rangeof its transmitted signal. The means for transmitting such a beacon orsignal are described in detail below. The receiving station(s) 12 mayreceive the beacon signal from the transmitters 10 and may associatewith such signals a time of arrival. Information regarding the signalwhich was received and the time of its arrival may be communicated bythe receiving station(s) 12 through conventional means to a centralstation 14 for use in a variety of ways, as described below. In oneembodiment of the present invention, the receiving station(s) 12 may becoordinated in time through the receipt of a clocking signal 22 from acommon source, such as the satellite system 24.

The signals transmitted by the transmitters 10 may include anidentification of the specific transmitter 10 which sent the signal, anindication that one or more events have occurred at the transmitter 10,a data portion relating to an activity or condition at the transmitter10 (such as, without limitation, a temperature, a flow rate, a pressurereading, etc.), an indication that emergency assistance is required atthe transmitter 10, and practically any other condition, indication,information, or circumstance which may be digitally encoded.

When the signals transmitted by the transmitters 10 are received at thereceiving stations 12, information regarding the signals and their timesof arrival may be communicated to the central station 14 for furtheranalysis. Depending upon the type(s) of signals being communicated, thecentral station 14 may store related information on the storage unit 18,may transmit information regarding the signals to other central stations14 or to other systems (not shown). For example, if the signalsrepresent events which have occurred at a transmitter 10, informationregarding the occurrence (and time) of the event may be sent to anothersystem for operations or control purposes. Such a system could include adetector which detects an improper entry into a building and triggers anevent signal at a transmitter 10. When the entry event is received bythe central station 14, the central station 14 may notify a local policedepartment of the event and the location of the transmitter 10 forappropriate police response. For another example, the signals couldinclude data from a medical sensor attached to a user of the transmitter10. When passed to the central station 14, the signals could be used todetermine the present health of the user or to record (using the storageunit 18) the physical characteristics of the user over time. If theuser's health were determined to need assistance, the signals from thereceiving stations 12 could be analyzed, as discussed below, todetermine the geolocation of the user so that medical personnel could bedirectly dispatched. Finally, the signals could merely identify thetransmitter 10 and its location. Such a system could be used, forexample, to monitor the instantaneous or history of the location of eachtruck in a fleet of delivery trucks.

In one embodiment of the present invention, the signals from thetransmitters 10 may have been encoded with digital information(signifying the identification of the transmitter, events, data, etc.).One means of encoding such data may be as illustrated at FIG. 2 in whichchirp signals are generated at a predetermined rate and the second andsubsequent chirp signals (or "chirps") are initiated at an offset fromthe time at which the predetermined rate would indicate. As shown inFIG. 2, a first chirp signal may start at any arbitrary start time, t₀.A second chirp would be expected to start at a predetermined time afterthe start time, depending upon the predetermined rate. To encodeinformation, the delay of the second and subsequent chirps in a group ofchirps can be delayed by detectable amounts and the amount of the delaycan be made a function of the data to be transmitted. For example, withrespect to FIG. 2, a chirp can be generated at a rate of 100 chirps persecond. If the second and subsequent chirps are initiated at a delay ofX microseconds, where X is a number between 0 and 31, each subsequentchirp can represent 5 digital bits of information (2⁵ equals 32). Iffive chirps are sent as a group, all timed from the initiation of thefirst chirp, the entire group of chirps can encode twenty digital bitsof information (4 subsequent chirps, each capable of encoding 5 bits ofinformation). The types of information which may be encoded is broad andmay include an identification of the transmitter, a sensor reading, aevent identification, etc.

With continued reference to FIG. 2, the timing of the initiation of thesecond chirp equals 10 milliseconds (the duration of the first chirp)plus N microseconds, where N is a number between 0 and 31 and represents5 bits of data to be transmitted. Likewise, the start of Chirp 3 willoccur at 20 milliseconds plus M microseconds, where M is a numberbetween 0 and 31 and represents another 5 bits of data to betransmitted. Upon the completion of the transmittal of five chirps, thesequence of transmission can be restarted at any arbitrary timethereafter.

With reference to FIG. 3, a chirp signal as used herein is a signalhaving a continuously varying frequency over a finite period of time. Asdepicted in FIG. 3, a chirp signal used in the present invention may bea signal having 4 MHz bandwidth which is swept over a 10 millisecondperiod. While the sweep of the chirp is shown in FIG. 3 as being upwardover time, the present invention is not so limited and the chirp maysweep downward in frequency. It is also desirable that the sweep belinear, that is, the plot of the frequency of the signal over time is astraight line. The use of a linear sweep permits the receiver of thesignal to use a time invariant matched filter to decode the transmittedsignal, even with a signal that has a relatively large frequency offset.Thus, the transmitter may be built using a relatively inexpensive timingsource, such as an inexpensive oscillator with poor frequency stabilityand the system will perform satisfactorily.

As will be recognized by those skilled in the art, the exact timing ofthe length of the chirps, the number of chirps in a group, the number ofbits encoded by the delay in initiation of a chirp, or the amount ofdelay in the initiation of a chirp associated with a particular bitsequence are not limited to the times and numbers used in the exemplaryembodiment and can be set to any practical values depending upon thesensitivity and accuracy of the transmitting and receiving equipment.

With reference now to FIG. 4, a transmitter in accordance with oneaspect of the present invention may include a timer 36 which providestiming signals to a controller 38 and to a power control circuit 40. Thecontroller 38 may also receive signals which provide information to thecontroller 38 such as the ID (identification) of the transmitter, a datareading from a SENSOR, a signal indicating the use of a PANIC button onthe external casing of the transmitter 10, or a signal indicating theoccurrence of an EVENT or one of several predefined EVENTs. Based on thetiming signals and the information signals, the controller 38 mayinitiate a chirp generator 41 to generate a chirp signal at a desiredinstant of time. The chirp signal is provided to one of the inputs of amodulator 42 which modulates a carrier signal received from a carriersource 44 with the chirp signal to provide a modulated signal. Themodulated signal is provided to a power amplifier 46 and ultimately toan antenna 48 for propagation to one of the receiving stations (notshown).

In operation, the timer 36 provides timing signals to the controller 38and the power control circuit 40. The timing signals are generated toprovide the timing within a group (or "frame") of chirp signals orbetween consecutive groups of chirp signals in accordance with theparticular signalling protocol used by the transmitter. The timingsignals are also used to energize and deenergize other portions of thecircuitry of the transmitter when such portions are not needed. Forexample, the timing signals, operating through the power controlcircuit, may cause the chirp generator 41 and transmission circuits(modulator 42, power amplifier 46, carrier source 44, etc.) to bedeenergized when no transmissions of signals are occurring.

With continued reference to FIG. 4, the controller 38 uses the timingsignals to encode the various information desired to be transmittedusing the encoding scheme of chirp signal position encoding discussedabove in relation to FIG. 2. The information signals are received from avariety of sources, depending upon the specific information transmissionneeds of a particular system. An identification signal (ID) may beprovided by switch settings, ROM programming, or a similar method ofproviding data which is not harmed by the removal of power from thetransmitter. A PANIC signal may be initiated by the user of thetransmitter through the operation of a panic button on the housing of atransmitter or through the entry of a particular key sequence on akeyboard associated with the transmitter. The SENSOR data may bereceived from any measuring, reporting, indicating unit which provides adata signal to be transmitted. The EVENT signals may be any type of datasignal which indicates the passage of an event or group of events,including without limitation, buttons, switches, logic circuits, keys,etc. The signals from any of the sources may be in analog, digital orany other format recognized by the controller 38 and (if needed)convertible by the controller 38 to a digital signal.

At the appropriate time, depending upon the application, the controller38 causes chirps to be generated by the chirp generator 41 to encode thedata desired to be transmitted. The data may be as simple as theidentification of the transmitter which is transmitted on a periodicbasis or upon the occurrence of a particular event or sensor reading. Inmore complex systems, the data to be encoded and transmitted may includeboth an identification signal and the reading(s) from one or moresensors or event indicators. The framing of the encoded signals into onegroup of chirp signals or more than one group of chirp signals may beaccomplished as needed for the length of the information to be sent. Thegenerated chirps are used to modulate a carrier signal received from thecarrier source 44 to provide a modulated signal which is amplified bythe power amplifier 46 and transmitted through an antenna 48 in aconventional fashion. The modulated signal may be filtered, predistortedor otherwise manipulated as is conventionally known and well within theskill of the artisan for the amplification and transmission of modulatedsignals in general.

The carrier source may be any convention source of a carrier signal,such as, without limitation, a crystal oscillator, a digital signalsynthesizer, an analog resonant circuit, or a signal provided by anexternal source. The chirp generator 41, modulator 42, carrier source44, and related components may be either digital or analog devices.

With reference to FIG. 5, one embodiment of a receiving station of thepresent invention may include an antenna 50 having four separatereceiving elements 52. The signals from the individual receivingelements 52 are provided to a like number of channel processors 54 whichdemodulate the signals received on the elements 52, decode theinformation contained therein, and, in some instances, determine thetime of arrival and/or angle of arrival of the decoded signals at therespective elements 52. Information regarding the received decodedsignals and their times of arrival and/or angles of arrival may beprovided to a data processor 56, which may provide an indication of theidentity of the transmitter which has sent a signal, the data (if any)which was sent by the transmitter and, in some instances, the locationof the transmitter as determined from the time and/or angle of arrivalof the signals at the elements 54.

One system by which the signals received at the receiving stations maybe decoded is illustrated, broadly, in FIG. 6 in which an incoming chirp60 (after appropriate downconversion, filtering and amplification) ismodulated by a reference chirp signal 62 by a modulator 64 at thereceiving station. In practice, the reference chirp signal 62 shouldhave similar characteristics to the chirp signal which was generated atthe transmitter. Preferably, the reference chirp should match thereceived chirp signal in slope, that is the plot of frequency versustime for the signals should have the same slope. As is known, themodulation of a chirp signal by another chirp signal generates acontinuous wave signal ("CW") which has a frequency which is a directfunction of the difference between the timing of the start of the twochirp signals. If the two chirps input to the modulator 64 have exactlythe same timing, the output from the modulator 64 will be null. As thestart timing between the two chirp signals increases, the frequency ofthe continuous wave signal generated by the modulator will increase.

If the chirps are relatively linear (as determined above), a frequencyoffset between the incoming chirp and the reference chirp will haverelatively little effect on the performance of the system. If the chirpsignals are relatively linear, a frequency offset has the same effect asa time offset between the incoming signal and the reference signal. Asis explained below, the time differences between the signals from theremote transmitters and the receiving stations may be used to determineposition by comparing the time differences at different receivingstations. Since these calculations are always relative to one another,the apparent time offset is removed at the system level by differentialtime of arrival processing. Thus, it is particularly advantageous inattempting to minimize the cost of the transmitter (by using relativelyinexpensive timing circuits) to use a relatively linear chirp signal,which is a waveform known to have the above-described offset properties.

In the receiving stations of the present invention, the CW signal fromthe modulator 64 may be bandpass filtered to remove or discard signalsin which the reference chirp and the incoming chirp are not sufficientlyaligned in their initial timing. Signals passing through the bandpassfilter 66 may be digitized by a analog to digital converter 68 andanalyzed in a Fast-Fourier-transform ("FFT") circuit 70. The FFTdetermines the frequency of the signal received by it. Because thefrequency of this signal is directly related to the timing between the(local) reference chirp and the received chirp, the frequency determinedby the FFT will directly indicate the difference between the timing ofthe reference and incoming chirps, leading directly to both the time ofarrival of the incoming chirp and the delays in the initiation of thesecond and successive chirps in an incoming chirp group (i.e., to thedecoding of the data encoded by the time position of the chirps). Aswill be appreciated by those skilled in the art upon review of thesimplified circuit of FIG. 6, by adjusting the timing relationshipbetween the reference chirp and the incoming chirp, both the time ofarrival of the incoming chirp and the data encoded thereon may beobtained.

With reference to FIG. 7, a channel processor 54 may decode the receivedsignal and extract the information encoded thereon by receiving a signalfrom one of the elements 52 of the antenna (not shown). In conventionalfashion, the received signal may by low noise amplified and downconverted by LNA/CONV 72 to an intermediate frequency signal which issplit and provided to plural subchannel processors 74. Within asubchannel processor 74, the intermediate frequency signal is modulatedin a modulator 76 and provided to a bandpass filter 78. The filteredsignal is applied to an analog to digital converter 80 which convertsthe signal to a digital form which is then down converted by a digitaldown converter 82. The downconverted signal is provided to an FFT 84,the output of which is analyzed by a signal detector 86. The detectedsignal (if any) from the signal detector 86 is provided to a dataprocessor with, in some instances, a signal indicating the time ofarrival of the detected signal.

With continued reference to FIG. 7, the channel processor 54 may alsoreceive a clocking signal from a common source, such as a GlobalPositioning System ("GPS") satellite through a GPS decoder 88. Theclocking signal is applied to a reference chirp generator 90 whichgenerates a reference chirp similar in form to the chirp signal used bythe transmitter (not shown) but asynchronous to the transmitter. Thereference chirp signal is applied to a time offset circuit 92 whichprovides four copies of the reference chirp signal, each at a differentdelay from the other reference chirp signals. One copy of the referencechirp signal is supplied to the other input of the modulator 76 withineach of the subchannel processors 74.

In operation, the time offset circuit may delay the various copies ofthe reference signal by increments equal to 1/4 of the length of thechirp signal. For example, the copies of the reference chirp provided bythe time offset circuit 92 are delayed by 0, 1/4 the length of thechirp, 1/2 the length of the chirp and 3/4 the length of the chirp. Inthe example system as used above, having a chirp length of 10milliseconds, the time offset circuit 92 would produce reference chirpsignals having 0 seconds, 2.5 milliseconds, 5.0 milliseconds, and 7.5milliseconds offset, respectively, from the reference chirp. The fourmutually-time-offset reference signals are then provided one to each ofthe subchannel processors 74.

With continued reference to FIG. 7, once a received signal has beenamplified and down converted, the received signal is split into fouridentical signals which are provided to the modulators 76 of thesubchannel processors 74. At each of the subchannel processors 74, thereceived signals are processed similarly; however, different results areobtained because of the timing differences in the reference chirpsignals supplied to the subchannel processors 74. Within a subchannelprocessor 74, the received signal is modulated by the reference chirpsupplied to it by the time offset circuit 92. As explained earlier, andassuming there to be a chirp signal within the received signal at anasynchronous timing with respect to the reference chirp, the modulationof the received chirp signal by the offset reference signal willgenerate a CW signal having a frequency directly dependent upon thedifference in timing between the initiation of the received chirp signaland the offset reference chirp signal. The CW signal is filtered by abandpass filter designed to pass signals having frequencies within aparticular bandwidth of the intermediate frequency. Accordingly, thebandpass filter 78 removes those CW signals from received chirp signals(if any) which do not having an initial timing within a specific timerelationship to the offset reference signal. If the bandwidth of thebandpass filter 78 is sized appropriately, the bandpass filter 78 ofonly one of the subchannel processors 74 will pass the CW signal of aparticular received chirp signal, while the other bandpass filters 78 inthe other subchannel processors 74 will filter out the CW signalgenerated by the modulation of the received chirp signal by offsetreference signals not sufficiently matching the timing of the receivedchirp signal.

The isolation of the subchannel processor having the offset referencechirp signal best matching the timing of the received chirp signal isillustrated in the timing diagram of FIG. 8, along with FIG. 7. Thetiming diagram shows the generation of the four offset referencesignals, REF 1, REF 2, REF 3, REF 4, with the vertical lines in the REFsignals indicating the start of an offset reference chirp. If thebandwidth passed by the bandpass filter 78 is equivalent to a delay of+/- one-eighth of the length of the chirp, received chirp signals (onlytwo shown) having the timing indicated by reference numeral 100 will bethe earliest signals passed by the respective bandpass filters andreceived chirp signals having the timing indicated by reference numeral102 will be the latest signals passed by the bandpass filters. Note thatregardless of the timing of a received chirp signal, it will be passedby one of the bandpass filters and rejected by the other bandpassfilters. Note also that the length of time "C" from the earliestreceived chirp at any one of the subchannel processors to the latestreceived chirp at any one of the subchannel processors matches thelength of the chirp. As will be appreciated by those skilled in the art,practical problems in building bandpass filters with precise and sharpcutoff edges will result in some situations in which the bandpassfilters of two of the subchannel processors 74 may permit their CWsignals occurring near the extremes of the bandwidth to pass through;however, such practicalities do not substantially adversely affect thepresent invention as the subsequent processing of the signals can removeor resolve any ambiguities in signal timing caused thereby.

With reference again to FIG. 7, in the subchannel processor 74 having aCW signal which passes through its bandpass filter 78, the CW signal isconverted to its digital form by the analog to digital converter 80 andis digitally down converted to baseband by the digital down converter82. The downconverted CW signal having a frequency related to thedifference in timing of the offset reference chirp and the receivedchirp is provided to an FFT which determines the frequency of the CWsignal. If the FFT determines that there is a CW signal present duringany particular chirp period, the signal detect circuit 86 provides asignal to the data processor 56 indicating both the presence of the CWsignal and the frequency thereof (which can be directly related to thetiming of the received chirp signal with respect to the appropriateoffset reference chirp signal).

With continued reference to FIG. 7, in the exemplary system discussedabove, having a chirp length of 10 milliseconds, it has been foundadvantageous, although not required, that the intermediate frequency ofthe signal input to the subchannel processor 74 be approximately 70 MHz.For the exemplary chirp signal having a bandwidth of 4 MHz, a bandwidthof 1 MHz in the bandpass filter 78 may be used.

It will be appreciated by those skilled in the art that the number ofsubchannel processors 74 is not limited to four, as used in theexemplary system. More or fewer subchannel processors 74 may be usedwith proper adjustments in other portions of the circuit such asadjustments for the amounts by which the reference chirp is offset andthe bandwidth of the bandpass filters 78 with respect to the bandwidthof the chirp signal being used in the system.

With reference to FIG. 9, the operation of the FFT may be readilyunderstood. As is well known, the output of an FFT may be considered asequence of "bins". Each bin represents a frequency range and togetherall of the bins represent the bandwidth of the FFT. The FFT depositsinto each bin a count (derived from the inphase and quadraturecomponents I² *Q²) indicative of the relative amplitude of thefrequencies contained in the signal applied to the FFT. Since, ingeneral, signals can have multiple frequencies present, more than onebin of an FFT may have significant counts present, each significantcount representing the fact that the input signal has the frequencycomponent related to the specific bin. In one embodiment of a system ofthe present invention, the output bins of the FFT in a subchannelprocessor having an acceptable CW signal may have counts as illustratedin FIG. 9. Note that most of the bins have some counts presence,illustrating the presence of broadband noise. More significantly, one oradjacent bins will have counts significantly higher than the noise bins,indicating the presence of a CW signal having the frequency associatedwith that bin(s).

In the exemplary embodiment of the present system, only CW signalshaving a bandwidth less than 1 MHz will pass the bandpass filter.Accordingly, the bandwidth of the FFT does not need to be greater than 1MHz. If the FFT has 10,000 bins, each bin has an associated frequencywidth of 100 Hz (i.e., 1,000,000/10,000). Because each subchannelprocessor in the exemplary system deals with 2.5 msec of time (a 10 mseclength of chirp divided into four subchannels), each bin of the FFTrepresents 250 nanoseconds of time offset between the reference chirpand the received chirp signals which yields an approximate 250 footrange of resolution for the system when the time of arrival of thereceived signal is used to compute a location of the transmitter of thesignal.

While the foregoing system has been described as identifying a singlereceived chirp signal within a 2.5 millisecond period (the period oftime represented by one subchannel processor), the present invention isnot so limited. The signal detect circuit could select not just thestrongest signal within the output of the FFT but all signals having acount in excess of a desired value or criterion. By using such a signalselect circuit, the present invention can accurately detect and decodethe signals from plural, asynchronous transmitters all of which haveinitiated their transmissions in such a fashion that the signals arrivedat the receiving station within 2.5 milliseconds of each other.

Likewise, for ease of explanation of the subchannel processor logic, theforegoing description of a preferred embodiment has described four,parallel subchannel processors; however, if components of sufficientspeed and additional memory is provided, some of the devices could beshared by the different subprocessors. For example, in a particularembodiment of the present invention, a single FFT could be used andswitched between the subchannel processors. Other similar changes toshare circuitry will be perceived by those skilled in the art withoutdeparting from the present invention.

With reference again to FIG. 7, the signal detect circuit 86 detects andidentifies a received chirp signal or signals once each chirp cycle andpasses this information along with its or their time of arrival to thedata processor 56. The data processor 56 stores and processes the chirpdata over plural chirp cycles, the number of cycles being dependent uponthe protocol and framing of the system being implemented. For example,in the exemplary system described herein, the data processor 56processes the last five chirps (i.e., the length of a chirp group offrame). As illustrated in FIG. 10, the data processor 56 datademodulates the chirp signals by comparing the results for the last five(in the exemplary system) chirps for each subchannel processor. Bysliding a 128 bin "window" across and down the output from onesubchannel, the data processor 56 detects the presence of a frame ofchirps (from the same transmitter). The width of the window may bedetermined from the encoding scheme used by the transmitter and the timerepresented by each bin of the FFT. For example, in the exemplarysystem, the encoding was done by delaying the initiation of the secondthrough fifth chirps by up to 32 microseconds from the period of thefirst chirp. In the exemplary FFT in which each bin represents 250nanoseconds of time, a delay of 32 microseconds equates to 128 bins (32microsec/250 nanoseconds per bin=128 bins). If the data processor 54detects five signal "hits" within the window, a frame of data has beendetected. The time of arrival of the chirp frame may be taken from thetime of arrival of Chirp 1 of the sequence of five chirps. Thedifference in bin numbers between Chirp 1 and Chirp 2 (divided by 4, asonly every fourth bin is used for data in the exemplary system) providesthe decimal number (between 0 and 31) representing the binary encodingof five bits. Similarly, the differences between the bins of Chirp 1 andof Chirps 3, 4, and 5 provide an additional fifteen bits of data.

By examining the outputs from all of the FFTs in the subchannelprocessors 74, the data processor 56 is able to assemble all of theframes of data (and the times of arrival) sent asynchronously by theplural transmitters within its range of reception. Thereafter, usingconventional data decoding techniques, the data processor 56 may respondto the data as appropriate for particular applications by relayingmessages to other systems, generating messages to be sent to othersystems, and, generally, taking actions appropriate to the data/message.

As noted earlier, in one embodiment of the present invention, the dataprocessor may receive signal data from plural channels (four in theexemplary system), each channel associated with a separate one of theantenna elements. In normal circumstances, signals reaching the antennawill be received at each of the elements, albeit at different times andwith different arrival angles. In decoding the data, the data processor56 may combine the results from the four channels by coherently addingthem together, by voting the results of the four channels, or by anyconventional results-accumulating scheme. If coherent totalling is used,false subchannel results stemming from noise will all non-coherently andwill be lost.

The use of plural channels also enables the data processor 56 to computean angle of arrival of a signal detected on plural elements in aconventional fashion.

In one aspect of the present invention, the signal data, time of arrivaldata and the angle of arrival data from plural receiving stations may becombined to identify and geolocate the transmitter(s) sending thereceived signals. As is known the time-difference-of-arrival of the samesignal at three diverse receiving stations may be used to geolocate thetransmitter of the signal. Similarly, the angle-of-arrival of the samesignal at two diverse receiving stations may be used to geolocate thetransmitter of the signal. Either or both the time-difference-of-arrivalor the angle-of-arrival or a combination of both techniques may be usedto geolocate a specific transmitter of the present invention.

Modifications to the disclosed system may be made within the spirit ofthe invention to provide data signals and/or geolocation information.For example, a system which used three receiving stations, each havingonly one channel, could be used to receive data messages and togeolocate the transmitter(s) of those messages based on thetime-difference-of-arrival of the signals at the three receivingstations. In such a system, there would be no requirement to compute theangle of arrival of the signals. Similarly, the present system can beused to communicate data (or events) and not to geolocate thetransmitter at a savings of the elimination of the multiple antennaelements at each station.

Still other waveforms may be used to transmit the data signals and to bematched at the receiving stations. For example, the transmitters coulduse a pulse position encoding technique, such as described above, toencode the identification and/or data which is to be transmitted andthen could modulate the data signal using a conventional direct sequencespread spectrum technique (such as a PN modulation technique). Uponreceipt, the data signal could be decoded using, first, a conventionalcorrelator decoder followed by a data demodulator which decodes thepulse position information. Similarly, the data signal can be generatedand transmitted as described above with reference to FIGS. 2-4 and thereceiving station can use a chirp signal correlator to determine thepresence and time of arrival of a valid data signal. In such acorrelator, the input signal may be correlated against a reference chirpsignal (another form of a time invariant matched filter from thatdisclosed earlier). In such a system, when the outputs from the matchingprocess of the correlator are aligned, an impulse is obtained,indicating the presence of at least one signal which had beentransmitted in accordance with the teachings of one aspect of thepresent invention.

The applications in which the features and advantages of the system ofthe present invention can be successfully utilized are many. Suchapplications include, without limitation: a fleet management system inwhich vehicles carry transmitters (or "tags") which periodicallytransmit an identify signal so that a central control system can monitorthe location of the fleet to ensure adequate coverage (e.g., fortaxicabs or police patrols), to assist in efficient dispatch, or toinhibit the use of vehicles on unauthorized trips; a personnel locationsystem for parolees in which the parolee carries the tag on his personto ensure he remains in authorized locations; surreptitious tracking ofsuspect vehicles using hidden tags attached thereto.

In contrast to the prior art, the present invention provides a systemfor communicating data and for geolocating plural objects in arelatively inexpensive system. The transmitters of the present inventionare not synchronized either to each other, to the receivers or to anyother infrastructure. Further, the transmitters do not need to have theability to receive signals (as in, for example, polling systems) inorder to permit multiple transmitters to be able to transmit nearlysimultaneously with a substantially reduced likelihood of disastroussignal. The receiving stations in the present invention do not needapriori knowledge of the identity of the transmitter before decoding amessage which has arrived at an unknown time.

In further contrast with the prior art, the present invention permitsasynchronous, simultaneous transmissions by plural transmitters but doesnot require a complex and costly set of correlators in each receivingstation in order to sort out the plural signals from one another. Byusing a time invariant matched filter, the present invention avoids theneed in prior art systems for polling and/or for complicated receivingstations which are capable of looking within small signalling periodsfor one of a large set of possible signal positions.

As will be readily seen by one skilled in the art, the relatively lowcost of the transmitters of the present invention, coupled with theabsence of any need for forward transmissions to mobile units and therelatively high capacity of the system in terms of the number of tags(or remote transmitters) which may be simultaneously operating therein,and the ability to transmit both identifying information and other datafrom the tags provide a flexible system and method of efficientcommunications.

While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the invention is to be defined solelyby the appended claims when accorded a full range of equivalence, manyvariations and modifications naturally occurring to those of skill inthe art from a perusal hereof.

What is claimed is:
 1. A locating system for determining the location ofa transmitter comprising:(A) plural receivers, each of said receiverscomprising:(a) means for receiving a radiated signal containing digitaldata modulated thereon; (b) a down converter for down converting saidreceived radiated signal to a lower frequency signal; (c) a chirp signalgenerator; (d) a mixer which periodically mixes said lower frequencysignal with a chirp signal received from said chirp signal generator toproduce a wave signal having a frequency related to the timerelationship between the start of the chirp signal and the start of thedigital data; (e) a filter circuit for filtering said wave signal toremove wave signals outside a predetermined range of frequencies; (f) afrequency detector for determining the frequency of the filtered wavesignal; (g) a detector for decoding the digital data within the filteredwave signal from the time relationship between successively demodulatedfiltered wave signals; and, (B) control means comprising:(h) means todetermine the time of arrival of a common one of said successivelydemodulated filtered wave signals received at each of said pluralreceivers; (i) means for determining the location of said transmitter bycomparing said times of arrival at said plural receivers.
 2. A systemfor locating a mobile transmitter, comprising:(a) plural means forreceiving transmitted signals, said transmitted signals having a datasignal intermittently modulated therein, each of said means forreceiving being spatially diverse from each of the others of said meansfor receiving, each of said means for receiving comprising:(i) pluralsubchannel processors, each processor comprising:(A) a mixer for mixingthe received signal with a periodically generated chirp signal toprovide an IF signal having a frequency related to the coordination intime between the start of the periodically generated chirp signal andthe data signal modulated within the received signal; (B) a filter toremove IF signals not within a predetermined bandwidth to provide afiltered IF signal; (C) a analog to digital converter to convert thefiltered IF signal from analog to digital format; (D) a digital downconverter to down convert the digital filtered IF signal to a basebandsignal; and, (E) a fast-Fourier-transform ("IFFT") processor to identifythe frequency components within the baseband signal; (ii) a chirp signalgenerator to periodically generate a chirp signal; (iii) a chirp signaltiming control circuit to apply said generated chirp signals to saidmixers within said subchannel processors in a timed sequence; and, (iv)a data demodulator which decodes the digital data within the receivedsignal by measuring the time relationship among the modulated datawithin the received signal; and, (v) time of arrival detector to retainthe time of arrival of the modulated data; and, (b) location determiningmeans which utilizes the time of arrival of the modulated data at theplural means for receiving to determine the location of the transmitterrelative to the location of the means for receiving.
 3. The system ofclaim 2 wherein said FFT processor develops plural frequency bins, eachof said bins being associated with signals having a different frequencywithin the bandwidth of signals passed through said filter.
 4. Thesystem of claim 3 wherein valid digital data are present in thefrequency bins if successive decoded digital data occur within apredetermined frequency offset within the frequency bins from the binholding the first decoded digital data.
 5. The system of claim 3 whereinsaid FFT produces 10,000 frequency bins and wherein valid data occurswithin a window of 128 frequency bins of successive chirp signals. 6.The system of claim 2 wherein plural digital bits of information areencoded within each modulated signal.
 7. The system of claim 2 whereinthe frequency of said baseband signal is related to the timerelationship between the periodic chirp signals and the intermittentlymodulated data signals.
 8. The system of claim 2 wherein five successivechirps form a frame of data, each frame having a base chirp and foursuccessive chirps, the timing between the start of the base chirp andeach of the successive chirps indicating the digital data beingrepresented by said frame.
 9. A transmitter location systemcomprising:at least one transmitter for radiating a signal containingdigital data modulated thereon; receiving means for receiving saidsignal, said receiving means comprising a chirp signal generator. 10.The system of claim 9 wherein said signal comprises a plurality of chirpsignals.
 11. The system of claim 10 wherein said receiving means furthercomprises:a plurality of antennae for receiving said signal; ademodulator for downconverting and decoding each of said chirp signalscontained within said signal; and, a data processor for further decodingsaid signal based on the timing of successive ones of said chirpsignals.
 12. The system of claim 11 wherein said demodulator comprises:aplurality of subchannel processors, each processor comprising: a mixerfor mixing the downconverted chirp signals with a reference chirp signalfrom said chirp generator to produce a continuous wave (CW) signal; afilter for filtering said CW signal; an analog to digital converter toconvert said filtered CW signal from analog to digital format; a digitaldown converter to down convert the digital filtered CW signal to abaseband signal; and, a fast-Fourier-transform (FFT) processor toidentify the frequency of said baseband signal.
 13. The system of claim12 further comprising means for generating multiple offset signals of areference signal provided from said chirp generator.
 14. The system ofclaim 13 wherein said demodulator comprises means for determining atiming relationship between each of said chirp signals and one of saidreference signals generated by said chirp generator such that saidtiming relationship is indicative of said digital data.
 15. The systemof claim 11 further comprising means for generating multiple offsetsignals of a reference signal provided from said chirp generator. 16.The system of claim 15 wherein said demodulator comprises means fordetermining a timing relationship between each of said chirp signals andone of said reference signals generated by said chirp generator suchthat said timing relationship is indicative of said digital data.
 17. Alocating system for determining the location of a transmittercomprising:at least one transmitter for radiating a signal containingdigital data modulated thereon; receiving means for receiving saidsignal, said receiving means comprising:a chirp signal generator; and, ademodulator for downconverting and decoding said signal.
 18. The systemof claim 17 wherein said signal comprises a plurality of chirp signals.19. The system of claim 18 wherein said receiving means furthercomprises a data processor for further decoding said signal based on thetiming of successive ones of said chirp signals.
 20. The system of claim19 wherein said timing of successive ones of said chirp signals isreferenced from the start of a first of said chirp signals.